In recent years, portable terminal devices such as smart phones with a digitizer, that is, a pen tablet function, have been commercialized and become popular in the market. The digitizer enables a user to draw a line of about 0.7 mm thick with a pen, and thus is more precise than a capacitive type touch panel recognizing a line of 3-4 mm thick, to thereby perform a fine work easily.
In addition, it is possible to take handwritten notes, to draw a picture, and edit an image or photo, by using a digitizer. Furthermore, pressure of a force applied to the pen is detected when a user holds the electronic pen in hand to write a letter and thus thickness of the letter varies depending on the detected force, to thereby enable a work with a high resolution.
To implement such a tablet function, a digitizer panel is provided at a lower side of a touch screen/display panel. In addition, since the digitizer panel is formed of a thin metal film, a feeble electromagnetic field is created when the digitizer panel conducts electricity, and since a built-in ultra-small metal coil is provided at the end of the portable wireless electronic pen, an alternating magnetic field is generated in use.
Thus, when the tip of the electronic pen is close to the touch screen, the electromagnetic induction phenomenon occurs, while deformation of the electromagnetic field that has already been formed occurs on the digitizer panel disposed below the touch screen/display panel. Here, deformation of the electromagnetic field is detected through a sensor arranged at a side edge of the digitizer panel to thereby be interpreted as the actual movement of the electronic pen.
This tablet function is being applied to a large screen tablet personal computer (PC) employing a large display as well as a small portable terminal device such as a smart phone.
In order to use a wireless electronic pen function using an electromagnetic induction phenomenon in a portable terminal device, a magnetic field shielding sheet for shielding an electromagnetic field generated from various components of a main body of the portable terminal device, is inserted and used between the digitizer panel and a main circuit board. The main body of the portable terminal device has a variety of communication chips and antennas and generates an electromagnetic field for wireless communications.
Recently, a spread of a long-term evolution (LTE) implementing a fourth generation mobile communication technology uses radio waves much stronger than a conventional wireless communication terminal using the 3G mobile communication system. Accordingly, an influence upon a digitizer panel from the strong electromagnetic field is precluded, and a reliable magnetic field shield is required for smooth magnetic field communications between the electronic pen and the digitizer panel.
Meanwhile, the portable terminal device includes a geomagnetic sensor in order to implement functions such as navigation or augmented reality by using GPS (Global Positioning System) technologies. In addition, in the case of smart phones employing the Android operating system (OS), it is an essential condition to adopt the geomagnetic sensor.
Since the magnetic field shielding sheet is formed of a size corresponding to a digitizer, i.e., a display, in order not to influence upon an electronic pen function, it is difficult to design a gap between the magnetic field shielding sheet and the geomagnetic sensor in the inside of the portable terminal device to become 2 mm or longer.
However, in the case that the magnetic field shielding sheet is used in the portable terminal device in the proximity of and together with the geomagnetic sensor, the magnetic field shielding sheet affects the geomagnetic sensor thereby causing the malfunction in the geomagnetic sensor.
In other words, the geomagnetic sensor may cause azimuth distortion, sensor sensitivity distortion, and magnetic hysteresis distortion by the magnetic field shielding sheet.
The azimuth distortion means a phenomenon of distorting direction of the magnetic north due to the magnetic field shielding sheet, the sensor sensitivity distortion means a phenomenon of distorting sensitivity among X-, Y-, Z-axis sensors constituting the geomagnetic sensor, since the strength of the magnetic field is also changing due to the magnetic field shielding sheet, and the magnetic hysteresis distortion means a phenomenon of making an error in the azimuth depending on the direction of rotation of a portable terminal device in which a geomagnetic sensor is mounted, because of the magnetic hysteresis of a magnetic substance.
Accordingly, in order to prevent the distortion of the geomagnetic sensor and in order to measure accurate azimuth, it is necessary to correct the geomagnetic sensor. However, it is possible to correct the azimuth distortion and the sensor sensitivity distortion accurately, through signal processing, but since it is difficult to correct the magnetic hysteresis distortion accurately, through signal processing, an error of the azimuth may exist according to the malfunction of the geomagnetic sensor.
It is general to use a magnetic substance such as Fe-based and Co-based amorphous ribbons, ferrite sheets, or polymer sheets containing magnetic powder, as the magnetic field shielding sheet. A magnetic field focusing effect to improve performance of a magnetic field shielding function and an electronic pen function, may be good in the order of high permeability Fe-based and Co-based amorphous ribbons, ferrite sheets, and polymer sheets containing magnetic powder.
The Fe-based and Co-based amorphous ribbons are metal sheets in themselves, and thus, have no burden to the thickness. However, the Fe-based and Co-based amorphous ribbons have too large magnetic permeability, and thus affect the geomagnetic sensor. As a result, the Fe-based and Co-based amorphous ribbons are not used as the magnetic field shielding sheets. In addition, the ferrite sheets also have too large permeability and thus affect the geomagnetic sensor. In addition, the ferrite sheets also have drawbacks of getting thicker.
Thus, the conventional magnetic field shielding sheet employs a polymer sheet containing magnetic powder of a relatively poor magnetic permeability. However, when compared with the Fe-based and Co-based amorphous ribbons, the polymer sheet has low magnetic permeability, to thereby cause a problem that a digitizer is very expensive, together with a problem that sensitivity of the digitizer is degraded to the half (½). However, the polymer sheet does not affect the geomagnetic sensor due to the low magnetic permeability, and thus is employed for a digitizer.
In addition, the polymer sheet containing magnetic powder has low magnetic permeability, when compared with the Fe-based and Co-based amorphous ribbons. In the case of increasing thickness of the sheet in order to improve the performance of the low magnetic permeability, the polymer sheet gets thick in comparison with the Fe-based and Co-based amorphous ribbons that are thin plates of several tens of micrometers (μm), together with a problem that the material cost is further increased due to an increase of the thickness. Accordingly, it is difficult to cope with the trend that the terminals get thin.
Korean Patent Laid-open Publication No. 10-2005-37015 discloses a metal and polymer composite having a low frequency magnetic field shielding function, wherein at least one selected from Permalloy®, Sendust®, and a rapidly solidified alloy that are metal alloys having the low frequency magnetic field shielding function is included by 10 to 80 wt % in a powdered, flaky or fibrous form; a soft polymer material is included by 15 to 65 wt % as a matrix where the metal alloys are dispersed; and various additives are included by 5 to 25 wt % in order to be used to mix the metal alloys and the soft polymer material.
The sheet proposed in Korean Patent Laid-open Publication No. 10-2005-37015 has a problem of low permeability when being used as a magnetic field shielding sheet for a digitizer, as a kind of a polymer sheet.
Meanwhile, Korean Patent Laid-open Publication No. 10-2011-92833 proposed an electromagnetic wave absorbing sheet containing a Fe-based nanocrystalline soft magnetic powder and a carbon-based conductive material powder. The Fe-based nanocrystalline soft magnetic powder is formed of a Fe—Si—B—Nb—Cu-based alloy as an amorphous alloy. The Fe—Si—B—Nb—Cu-based alloy is preliminarily heat treated at a temperature of 350° C. to 500° C. for 45-90 minutes, to thus obtain alloy powders, the alloy powders are primarily and secondarily crushed, and then the crushed powders are meshed to be 270 mesh or less in particle size, to thereby obtain Fe-based nanocrystalline soft magnetic powders having nano-sized crystal grains.
The electromagnetic wave absorbing sheet is made to have a thickness of 0.5 mm, to thus absorb 10 MHz to 10 GHz band electromagnetic waves.
However, the electromagnetic wave absorbing sheet is simply used to absorb the high frequency band electromagnetic waves, and employs a kind of the polymer sheet that is made to have a thickness of 0.5 mm by mixing a Fe-based nanocrystalline soft magnetic powder having nano-sized crystal grains, with a binder. As a result, the electromagnetic wave absorbing sheet gets thick when compared with the case of using an amorphous ribbon sheet (whose thickness is about 0.06 mm or less), and also has the low permeability due to the mixture of the binder.
Korean Patent Laid-open Publication No. 10-2003-86122 discloses a method of manufacturing an electromagnetic wave shielding material by using a metal foil ribbon of high magnetic permeability, the method including: preparing a metal foil ribbon in the range of 1 μm to 900 μm thick, and in the range of 1 mm to 90 mm wide, with a metal or alloy having a specific permeability of 1000 or more and selected from Ni—Fe—Mo, Fe—Si, and mu-metal by performing a quenching solidification method; annealing the metal foil ribbon in a temperature range of 700° C. to 1300° C. and under a hydrogen or vacuum atmosphere; and forming an adhesive layer on at least one surface of the metal foil ribbon.
Further, the electromagnetic wave shielding material manufacturing method further comprises forming a thin film layer of Cu, Ni, Ag, Al, Au, and Sn or a combination of these metals on at least one surface of the metal foil ribbon by electroplating or vacuum deposition.
However, the electromagnetic wave shielding material manufactured according to the Korean Patent Laid-open Publication No. 10-2003-86122 is not applicable to the magnetic shield sheet for a digitizer due to high magnetic permeability.
The aforementioned prior art discloses an electromagnetic wave absorbing sheet or a magnetic field shielding sheet. However, when both a digitizer function and a navigation function are implemented in a portable terminal device such as a smart phone, the above-described conventional magnetic field shielding sheet made of a Sendust® sheet or a heatless treatment Fe-based amorphous ribbon sheet does not propose a possible solution to the problems that sensitivity of the digitizer is low due to the low magnetic permeability, a distortion problem for the geomagnetic sensor is present due to too high magnetic permeability, or the material cost for the magnetic field shielding sheet is very expensive since the thickness of the magnetic field shielding sheet gets thick.
Taking into account that the distortion of the azimuth and the sensor sensitivity distortion among the distortions that occur in the geomagnetic sensor due to the magnetic field shielding sheet can be accurately corrected, but the directional distortion due to the magnetic hysteresis cannot be accurately corrected, the inventor(s) has tried to develop magnetic field shielding sheets that do not cause the magnetic hysteresis distortion problem. In addition, in the case that the Fe-based amorphous ribbon is heat treated to thus make low magnetic permeability, it is recognized that the geomagnetic sensor is not affected, which leads the inventor(s) to the present invention.
Meanwhile, the magnetic hysteresis means that a magnetic body has a hysteresis that magnetic induction values in the inside of the magnetic body do not match each other when a magnetic field is applied to the magnetic body through a rise and fall of the magnetic field, and the magnetic hysteresis may occur in the case that the magnetic field is applied to the magnetic body until the magnetic body is saturated. In the case that the magnetic field does not reach a saturation region, the magnetic induction values are repeatedly increased and decreased without causing any hysteresis along an initial magnetization curve.
In the case of the heatless treatment Fe-based amorphous ribbon, a value of a saturation field (Hs) that is the minimum magnetic field to obtain the saturation induction is about 0.4 G that is lower than the earth magnetic field having a value of about 0.5 G, in a magnetic hysteresis loop.
Therefore, the Fe-based amorphous ribbon sheet exhibits the magnetic hysteresis even in the case of the change in the earth magnetic field, with the result that a geomagnetic sensor used in a terminal to which a Fe-based amorphous ribbon sheet is applied, has a fatal disadvantage that even the magnetic hysteresis due to the Fe-based amorphous ribbon sheet should be corrected.
The present invention has been devised in consideration of facts that the magnetic hysteresis distortion problem can be blocked when the Fe-based amorphous alloy ribbon sheet is heat treated and flake treated to thereby increase a demagnetizing field so as not to form the magnetic saturation, the azimuth distortion and the sensor sensitivity distortion bearing on the geomagnetic sensor can be corrected by software, and the sensitivity of the electronic pen in the digitizer is preferable when the permeability of the shielding sheet is high.
Further, a nanocrystalline ribbon sheet having a nanocrystalline microstructure by a heat treatment exhibits a problem that uniformity of the permeability falls down in view of a whole sheet since the size of the nano-crystal grains in the heat treatment process fails to uniform, and such uniformity deterioration of the permeability may be a factor that degrades the uniformity of characteristics of a digitizer. On the contrary, if the Fe-based amorphous ribbon is heat treated below a crystallization temperature thereby having a tissue of an amorphous state, the characteristics of the tissue are uniform, and thus uniformity of the permeability is high in view of a whole sheet when compared with the nanocrystalline ribbon sheet having a large number of fine nanocrystalline structures, which led the inventor(s) to the present invention.
In general, in the case of the nanocrystalline ribbon sheet, it is known that it is difficult to prepare a ribbon of 50 mm or wider. Thus, when a magnetic field shielding sheet for a large-area digitizer of 50 mm or wider, it is difficult to secure the uniformity of the permeability in view of a whole sheet, since at least two sheets of the ribbons are butted or overlapped to cover a required large-area.
The conventional Fe-based amorphous ribbons have been used without passing through the heat treatment, or after undergoing the heat treatment. The main purpose of the heat treatment is to release the stress, through the magnetic field heat treatment for magnetic properties, for example, an increase in the permeability, a decrease in the core loss, or an increase in the saturation flux density, and through the embrittlement heat treatment for producing the powder, but the heat treatment of an object for reducing the permeability was not attempted.